The Application of isotropic bicelles as model membranes

Abstract: Isotropic bicelles are disc-shaped aggregates of lipids and detergents, and are suitable model systems for high-resolution NMR studies of membrane-interacting peptides. In this thesis the structures for the two peptides motilin and transportan were determined by homonuclear 1H methods in the presence of bicelles, and the structure of the bovine prion protein peptide (bPrPp) was solved in the presence of DHPC micelles. All of these peptides were found to be largely a-helical when bound to the model membranes. In subsequent experiments both motilin and transportan were shown to reside on the surface of the bicelles, whereas bPrPp is more likely to have a transmembrane configuration. NMR translational diffusion experiments revealed that the isotropic bicelles studied here are very large objects compared to what is regularly indicated by high-resolution NMR spectroscopy. Furthermore, these studies showed that all three peptides examined interact strongly with bicelles. Investigation of the NMR-relaxation of labeled sites in the peptides motilin and penetratin demonstrated that the overall rotational correlation times for these peptides do not reflect the bicellar size. Such decoupling of NMR relaxation from the dependence of overall size is also seen for the dynamics of the lipid molecules in the bicelles. It is therefore concluded that the overall size is not the sole determinant of the linewidths in NMR spectra, but that extensive motions within the bicelles also exert significant effects. Another interesting observation is that the membrane-bound structures of the peptides motilin, transportan, penetratin and bPrPp are very similar, even though these peptides have very different biological functions. In contrast, considerably more variation is observed in the membrane-positioning and molecular dynamics of these peptides. Since the bicelles have been found to induce differences in membrane positioning and molecular dynamics compared to micelles, these model membranes are likely to be important in order to enhance our understanding of the biological function of membrane interacting peptides.